The overall objective of the research planned and described is a systematic and detailed investigation of the interaction between early experience and genetic variables in the C57NL/10 mouse strain in an attempt to develop a more complete understanding of the mechanisms mediating between nature and nurture in the development of behavior. Specifically, the experiments are designed to determine whether the C57BL/10 uterine environment and/or X-chromosomal genes are required for effects of different early experiences in a variety of behavioral and biological traits. In the 1978-79 grant year, the following experiments are planned: (1) If there are strain-by-treatment interactions in DBA/1Bg and C57BL/10Bg strains for effects of fostering in the development of several behavioral and biological traits, the effect of this treatment on these traits will be determined in reciprocal F1 hybrids; (2) The effects of fostering and of early handling on the glucocorticoid stress response of DBA/1Bg and C57BL/10Bg mice will also be studied. If there are strain-by-treatment interactions, the effects of these treatments on the glucocorticoid stress response of reciprocal hybrids will also be studied; and (3) A study of the effects of fostering in control and ova transfer animals will be initiated to determine whether the C57BL/10 uterine environment and/or X-chromosomal genes are required for the developmental lability of C57BL/10 aggressive behavior.